Safety switching devices such as fail-safe sensors with the functions of a light curtain, a laser scanner, a two-hand pushbutton, a pressure mat, etc. are used for reliable field bus communication in process automation for example.
According to the prior art with fail-safe systems or during the fail-safe transmission of data from a safety switching device to a higher-order control unit telegrams with a backup appendix are sent in duplicate according to IEC 61 784-3 Annex A, Model B and C. Since here the information content is transmitted in a redundant manner in two telegrams on two different data channels or with a time offset one after the other on one data channel to the receiver or to the control unit, this transmission method has the disadvantage of a lower level of error detection in relation to data volume.
More extensive measures are therefore required to identify mutilated data. This could be done using a CRC checksum. The longer the checksum, the higher the level of error detection. However a long checksum lengthens the telegram, having an adverse effect on the response times of the control system.
Even with a CAN bus solution (CAN open safety) the fail-safe information is transmitted in duplicate (according to model C).